Semiconductor circuit components are normally produced by slicing an ingot of semiconductor material into thin wafers each of which are then processed to produce many individual integrated circuits. One of these processing steps involves introducing the wafers into a high temperature furnace to effect thermal oxidation, diffusion, CVD (chemical vapor deposition), annealing, etc.
In the case of diffusion treatment, a plurality of the wafers are placed on an intermediate quartz carrier which is commonly referred to as a "boat", and the boat is transported by loading apparatus from a loading station into one end of an adjacent diffusion furnace process tube. The wafers are subjected to a high temperature gaseous reactant within the furnace tube which precipitates out from the reaction in the form of dust-like particulates that accumulate along the sides and bottom of the tube.
In the event that the accumulated particulates within and around the tube become airborne and are deposited on the surface of the wafers, the deposited particulates result in defects such as pin-holes and other surface anomalies which render the integrated circuits defective. In order to reduce surface contamination due to these particulates, others in the past have devised a cantilever mounted paddle for loading and unloading the boats from the process tubes. One end of the paddle is mounted on a slidable carriage disposed outside of the process tube; one or more boats are loaded onto the opposite, free outer end of the paddle and the paddle is tipped in order to deposit or pick up the boat within the tube. Consequently, the outer end of the paddle does not contact the tube and thereby stir up particulates. Moreover, since only the outer free end of the paddle is inserted into the process tube, the amount of thermal mass introduced into the tube is minimized. A boat loader of the type described immediately above is disclosed in U.S. Pat. No. 4,468,195 issued Aug. 28, 1984 to Sasaki, et al.
Prior art boat loaders are less than completely satisfactory in a number of respects. For example, prior loaders, such as that disclosed in U.S. Pat. No. 4,468,195 discussed above, are normally mounted in stacked relationship within an enclosure at a loading station which is positioned adjacent the diffusion furnace. At least one wall of the enclosure is perforated to permit air flow over the wafers before and after they are processed in the furnace, thus providing a "scrubbing" action which prevents contaminating, airborne particles from coming to rest on the wafer surfaces. However, the airflow intended to produce the scrubbing action is substantially reduced because the relatively high profile of these loaders tends to block off the air flow.
Accurate positioning of the boats within the furnace tubes is particularly important, and the procedures required for calibrating prior loaders to effect proper boat displacement is not only time consuming but is also less accurate than is desired. Calibration is important because it is necessary to position the boat within a so-called "flat zone" within the furnace tube. The flat zone refers to the central portion of the tube which remains at a relatively constant temperature, unlike the ends of the tube which experience temperature variation due to the fact that the tube is essentially open at each end thereof.
Paddles of various configurations and different lengths are often employed to accommodate different types of boats; consequently, when the paddle on a loader is changed, it becomes necessary to recalibrate the control system for the loader in order to assure that the boat is delivered to the proper position within the tube. Prior control systems have relied on memory stored binary data constants representing limits on velocity, acceleration and displacement of the boat by the loader, however, these systems were of an open loop type in which the data did not accurately represent the actual velocity, acceleration or displacement of the boat. Consequently it was necessary for the operator to determine the recalibration data by a trial and error, iterative procedure.
Finally, in connection with prior art boat loaders, only the extreme limits of linear boat travel could be controlled, whereas, in contrast, it would be advantageous to provide control of linear boat travel to a number of individually selectable, arbitrary points between the extreme travel limits.
Each of the above mentioned problems are overcome by the soft landing boat loader of the present invention.